1. Field of the Invention
The instant invention relates to magnetic locks. More particularly, the instant invention relates to emergency exit door security systems wherein the system includes a magnetic door lock which senses when an attempt is made to open the door and/or senses when the lock is not secure.
2. Technical Considerations and Prior Art
The numerous patents listed above as "Related Patents" are generally directed to devices for delaying opening of emergency exit doors to prevent unauthorized exit from a building or room. Of particular interest with respect to the instant invention is U.S. Pat. No. 4,257,631, "Magnetic Emergency Exit Door Lock with Delayed Opening". As is readily apparent from the disclosure of this patent, initiation of a delayed time, opening interval is effected by pushing on the panic bar of the emergency exit door. The panic bar depresses a plunger of a switch, which switch in return is connected to time delay circuitry. Since the time delay circuitry is not disposed on the door, the line connecting the time delay circuitry to the switch must "jump the gap" between the hinged edge of the door and the door frame. In order to solve this problem, an "electric hinge" is utilized, which hinge has conductors and/or connectors therein for carrying current across the gap. These hinges are very expensive and are frequently difficult to select. This is because it is preferable that an electric hinge match the hinge set which a door is designed to accept. Frequently, a matching electric hinge is not available. In order to connect the switch to the time delay circuitry, a wire is usually run through the interior of the door to the electric hinge. Thereafter, the wire is run from the door jam side of the hinge through the door jam or wall to the circuitry. Running these wires is a time consuming, expensive operation.
In addition to the economic considerations, there are mechanical and safety considerations. There are numerous types of panic bars now on the market with which the switch must be mated. Accordingly, it is frequently difficult to configure the design and location of the switch so that it will operate with the particular panic hardware under consideration. Moreover, in many installations, the switch is exposed and therefore subject to damage and vandalism. Consequently, the switch may not operate properly when there is a need to open the emergency exit.
The assignee of the inventors named in the instant application has found that there is a market for magnetic locks both with the time delay feature and without the time delay feature. This is because it is frequently necessary, for one reason or another, to provide an indication that someone is attempting to open an emergency exit door. This indication may be used to start a time delay count in accordance with the principles disclosed and claimed in the aforelisted "Related Patents" or may simply be used to trip an alarm. For whatever reason, existing door security systems utilizing magnetic locks need a convenient, reliable and inexpensive way to provide such indication.
The prior art includes U.S. Pat. Nos. 4,487,439; 4,516,114 and 4,287,512 which address at least some of the concerns with which the instant invention deals. U.S. Pat. No. 4,487,439 discloses an electromagnetic lock with a floating armature which automatically adjusts for misalignments between the electromagnet and armature. However, there is no provision in this patent for sensing an attempt to open the door with which the lock is used, nor is there any provision for detecting whether or not the door is secure. Both U.S. Pat. Nos. 4,516,114 and 4,297,512, disclose provisions for sensing if a door equipped with an electromagnetic lock is secure. However, neither patent discloses a provision for detecting an attempt to open a door, which detection is necessary if a delayed opening arrangement is to be utilized. In U.S. Pat. No. 4,516,114, the current between two portions of the electromagnet is monitored to determine if the armature is in contact therewith. However, in this patent the security of the electromagnetic lock may be compromised by a sheet of aluminum foil. In U.S. Pat. No. 4,287,512 Hall cells are used to monitor magnetic flux in the electromagnet lock to determine if there has been or is an attempt to defeat the lock by taping or applying an external counter-magnetic field. However, with this arrangement there is no provision for detecting reduction of magnetic holding force due to skewing of the armature with respect to the electromagnet. Moreover, there is no suggestion in this patent of a provision for coordinating the operation of two types of sensors used with magnetic locks.